In the manufacture of most polymeric films, it is an object to adjust the composition and process conditions to produce a film which exhibits good mechanical properties such as increased tensile strength, elasticity and tear resistance. However, in certain applications, it is desirable to increase the potential destructibility of the film.
One such application relates to anti-counterfeit, anti-theft, security or “tamper-evident” labels. A particular area of application for such labels is in the packaging of consumer products, such as compact disks, computer software and pharmaceuticals; in “overlaminating” films; and in document protection. Such labels should be sufficiently brittle so that the labels fracture if an attempt is made to remove the label from the object to which it is adhered. A problem for retailers and consumers is the tampering with the packaging of an article. In some cases the packaged materials are altered and repackaged. It is desirable to have a label which would show that the article within the packaging had not been opened and/or altered. A suitably brittle label could indicate when a label or a package containing the label has been altered. This feature prevents the certifying label being removed from a product of guaranteed authenticity and subsequently being re-attached to a counterfeit. It also prevents a protective film from being removed from an underlying document or image so as to tamper with the document or image.
Previous attempts to produce brittle films having a low tear resistance have involved the use of low molecular weight polyesters in the manufacture of the film. However, such films are very susceptible to fracture during the film-forming process.
Acetate films having a low tear resistance have successfully been manufactured but have a tendency to absorb water and are not resistant to solvents such as water.
Polyester films are superior to acetate films in terms of water absorption and water resistance. JP-A-1229042 discloses a readily tearable film comprising a copolyester of ethylene glycol, terephthalic acid, a metal sulphonate group-containing aromatic dicarboxylic acid and either an aliphatic dicarboxylic acid or an aromatic dicarboxylic acid other than terephthalic acid, the film having a tear propagation resistance in at least one direction of 300 g/mm or less.
U.S. Pat. No. 4,746,556 discloses a tamper-evident label comprising a surface layer, an easily breakable layer laminated to the surface layer, a first peel-off layer formed on the surface of the easily breakable layer, a second peel-off layer formed on a part of the main surface of the easily breakable layer, a covering layer and an adhesive layer. The surface layer is peeled off the easily breakable layer and cannot be re-adhered.
U.S. Pat. No. 5,042,842 discloses multi-layer labels, such as those used for preventing or detecting tampering and counterfeiting, which has a base layer with a permanent adhesive coated on a lower surface. Indicia or markings are applied to the bottom surface of the top layer and the top layer is laminated to the upper surface of the base layer. The top layer is laminated to the upper surface of the base layer by permanent patterned adhesive. A portion of each of the upper surface of the base layer and the bottom surface of the top layer are free of adhesive.
WO-A-98/01291 discloses a multi-layer tamper-evident label comprising a first and second layer of polymeric film, wherein the lower surface of the first layer contacts the upper surface of the second layer at a separation interface; a heat-sealable layer wherein the lower surface of the heat-sealable layer is bonded to the upper surface of the first layer; and an adhesive layer wherein the upper surface of the adhesive layer is bonded to lower surface of the second layer.